(1) Field of the Invention
The present invention relates generally to the fabrication of color light detectors. More particularly, the present invention relates to the formation of protective coatings on filters used in color light detectors.
(2) Description of Related Art
Color light detectors are becoming increasingly important in digital imaging applications. Digital imaging systems often use arrays of photo-detectors to generate an image of a subject. In order to generate a color digital image, color filters are fabricated over light sensors such as photo-detectors. Each filter allows only a predetermined color of light (light in a particular frequency range) to reach a corresponding photo-detector thus determining what color light will be sensed by the photo-detector.
Each color pixel in a digital image is typically generated by combining the output of a set of photo-detectors. In one implementation, the output of three photo-detectors in a set are combined to generate one color pixel. Each photo-detector in a set is located in close proximity to the other two photo-detectors in the set. Each photo-detector of the set receives light filtered by a different colored color filter. In one example of a set, a blue color filter filters light reaching a first photo-detector, a red color filter filters light reaching a second photo-detector and a green color filter filters light reaching a third photo-detector. By determining the intensity of light passing through each color filter, the intensity of light of a particular color or wavelength can be determined. An electronic processor interpolates the data from the three photo-detectors and combines them to determine the color of light received by the photo-detectors in the general region of the pixel. This information is processed electronically and combined with other sets of photo-detectors to generate a digital color image.
Photo-detectors and color filters are typically formed in a complimentary metal oxide semiconductor (CMOS) fabrication process. A number of effects occur during the fabrication process that reduce the filtering capability or damage the color filters. In particular, three problems faced by the fabrication process are described in the following three paragraphs.
A first problem is color bleeding of compounds from adjacent color filters. Color filter arrays which are placed over the photo-detectors or imaging sensors are often generated by depositing pigment dispersed polymer films. The type of pigment determines the filtering capability of the filter. In a typical color detection system, adjacent filters have different pigments. The performance of the system is optimized when each photo sensor is covered with a single color filter, whether it be red, blue or green. The filter blocks other colors from passing through to the photo-sensor. Ideally, it is desirable to fabricate filters which transmit all of the light of a particular color and completely block other colors of light. Thus, for example, red pigments that determine the color of light that passes through a red filter are preferably completely confined to the red filter. In practice, the close contact between different color filters (red, blue, green) results in a slight intermixing of pigments ("bleed") during the fabrication process. The bleed results in a broadening of each individual filter response reducing the color delineation capabilities of each filter. This bleed degrades the overall performance of the system.
A second problem with current fabrication techniques is that during a grinding and gold deposition process (grind/gold process), color filters are often damaged. After the final deposition of CFA (color filter array) layer materials, wafers or substrates containing the color filters are transferred to a grind/gold process where a protective front side tape is applied to the wafer while the backside of the wafer is thinned and coated with gold. After completion of the grind/gold process, the tape is removed in a detaping operation. The filters are vulnerable to physical damage during the detaping operation. Damage to the color filters jeopardizes the functionality of the fabricated filter and photo-sensor combinations.
A third problem with current methods of fabricating filters results from repeating processing steps on the entire filter set each time a filter of a different color is added. After generation of a first color filter, each subsequent different color filter is produced by a subsequent deposition of a different pigmented polymer film in a subsequent layer. Thus a three color set (red, green and blue) of color filters involves three depositions of pigmented polymer films. Because each layer of pigmented polymer films is composed of similar materials, a misprocessing in the working or processing of a layer may damage previously created layers and require reworking of previously deposited layers.
Thus a method or apparatus of avoiding the above described problems is needed.